A motion boat covers a given distance in 6 h moving down stream of a river. It covers the same distance in 10 h moving upstream. The time (in hour) it takes to cover the same distance in still water is

Let $v_w$ be velocity of water and v_b be the velocity of motor boat in still water. If x is the distance covered, then as per question

$x=\left(v_b+v_w\right)\times 6=\left(v_b-v_w\right)\times 10$

On solving, $v_w=v_b/4$

$\therefore x=\left(v_b+v_b/4\right)\times 6=7.5 v_b$

Time taken by motor boat to cross the same distance in still water is

$t=\frac{x}{x_b}=\frac{7.5}{v_b}=7.5 h$